1. Field of the Invention
The invention relates to an electric motor.
2. Discussion of Background
Conventionally, there has been an electric motor that includes a so-called “embedded magnet rotor” in which permanent magnets are embedded in a rotor core so that the permanent magnets are fixed to the rotor core (for example, Japanese Patent Application Publication No. 2010-233346 (JP 2010-233346 A)). In an electric motor including such an embedded magnet rotor, in addition to magnet torque by permanent magnets, reluctance torque is generated. Therefore, the electric motor including the embedded magnet rotor has an advantage over an electric motor including a so-called “surface magnet rotor” in which permanent magnets are fixed to a surface of a rotor core, in that the electric motor including the embedded magnet rotor is able to generate higher torque.
In order to increase magnet torque, it is necessary to increase an amount of magnetic flux that is transmitted between a stator and the rotor, in other words, a magnetic flux density on an outer periphery of the rotor core. However, employment of permanent magnets having a larger maximum energy product is only one effective measure to increase the magnetic flux density on the outer periphery of the rotor core in the above-described conventional configuration. Therefore, there has been a demand for creation of a new technology that makes it possible to increase a magnetic flux density on an outer periphery of a rotor core regardless of the performance of permanent magnets embedded in the rotor core.